Switches in data communication systems transfer a plurality of communication spans from a line facility to a drop facility, and vice versa. These spans typically contain a plurality of time division multiplexed channels. Any channel within a span at one of the drop and line facilities may be selectively connected within the switch to any channel of any span at the other of the facilities. Consequently, a switch may support a considerable amount of communication and may contain a relatively complex map that defines a current interconnection scheme between the channels supported by the switch.
Upon occasion, a user of an existing switch may desire to expand its capability. In other words, the user may wish to install a replacement switch having the ability to switch a greater number of channels between drop and line facilities than are currently supported by the existing switch. However, it is usually undesirable to simply de-activate the existing switch, disconnect the existing switch, connect a replacement switch, activate the replacement switch, and establish the map that defines the interconnection scheme in the replacement switch. Such a replacement procedure ends all data communications service for an intolerably long period of time until the replacement switch is fully on-line.
Conventional telecommunication industry standards require equipment and users of communication facilities to tolerate occasional short duration service interruptions. However, long duration outages may severely harm a user of such communication facilities. A loss of service for greater than a few seconds is generally considered service-affecting. Accordingly, it is highly desirable to minimize the possibility of service-affecting interruptions. Moreover, if service-affecting interruptions are unavoidable, then an interruption which affects only a few channels is desirable over a service-affecting interruption which affects all channels supported by an existing switch.
Additional switch hardware may be employed to minimize service-affecting interruptions that might occur as a result of switch replacement. However, such additional hardware may undesirably impact system cost and reliability and seldom is capable of switching large amounts of data communications within an acceptable time frame. Furthermore, the additional equipment must be removed after the switch over to the new switching equipment is completed. Therefore, a need exists for a replacement switch and a procedure for replacing an existing switch which minimizes service interruptions and which minimizes the amount of hardware that is dedicated solely to the replacement procedure.